Vacuum tube for generating a wide beam of fast electrons

ABSTRACT

A vacuum tube for producing a wide high-uniformity beam of fast accelerated electrons. The tube comprises an evacuated enclosure provided with an electron transparent window and containing an elongated cathode, accelerator electrodes and a control electrode formed by sleeves electrically insulated from one another and brought to separately adjustable potentials.

United States Patent 11 1 1111 3,780,334 Leboutet Dec. 18, 1973 [5 1 VACUUM TUBE FOR GENERATING A 3,105,916 /1963 Marker et 211...... 313/ x WIDE BEAM 0F FAST ELECTRONS 1,696,103 12/1928 Seibt 313/299 Inventor: Hubert Leboutet, Paris, France Assignee: Thomson CSF, Paris, France Filed: May 31, 1972 Appl. No.: 258,440

Foreign Application Priority Data June 9,1971 -France..,.l 7120938 US. Cl 313/299, 313/35, 313/74, 1

1m. (21. 11011 29/46, H01 j 33/00 Field of Search 313/299, 5, 6, 22, 1 1 313/35 References Cited UNITED STATES PATENTS 3/1968 Leiss et a1 .1 313/22 X FOREIGN PATENTS OR APPLICATIONS 108,947 10/1939 Australia 313/299 Primary Examiner-J0hn K. Corbin A tt0rney-Cushman, Darby 8t Cushman [57] ABSTRACT A vacuum tube for producing a wide high-uniformity beam of fast accelerated electrons. The tube comprises an evacuated enclosure provided with an electron transparent window and containing an elongated cathode, accelerator electrodes and a control electrode formed by sleeves electrically insulated .from one another and brought to separately adjustable po' tentials, 1

7 Claims, 6 Drawing ll igures PATENTED DEC 18 I975 SHEET 1 OF 3 VACUUMTUBE FOR GENERATING A WIDE BEAM OF FAST ELECTRONS In radiochemistry, and more particularly in the context of processing of paints, thin polyethylene films or expanded polyethylenes, it is frequently necessary to provide electron beams, the currents of which are high and uniform over a substantial width.

Although the acceleration energy of the electrons may not be of critical importance for applications of this kind, since the thickness of the material being processed is small, nevertheless sufficient energy is re quired for the electrons to exit from the window of the evacuated enclosure where they are produced, that is tosay for them to be able to penetrate some tenths of a millimetre of material and a thin gas film.

It is well known that the depth of penetration of an electron beam is i 65 mg/cm at 300 kV I03 mg/cm at 400 .kV

I40 mg/em at 500 kV.

Taking into account the thickness of the window and of the gas film separating said window from the object being bombarded, it is desirable that the acceleration of the particles should be at least 400 kV if they are to reach the target with sufficient energy.

The wide electron beam generator tube which forms the subject of the present invention, in meeting these requirements, provides a solution to the problem set out hereinbefore.

In accordance with the invention, a generator tube for producing a wide beam of fast electrons, comprises a vacuumtight enclosure and in said enclosure, at least one elongated cathode for emitting an electron beam throughout its length, a control electrode for controlling the form and the homogeneity of said beam, said control electrode comprising an elongated gap along which said cathode extends and through which said beam propagates said control electrode comprising a plurality of electrode elements, electrically insulated from one another means for respectively connecting said elements to a voltage supply means for accelerating said beam said enclosure comprising an elongated electron-transparent window for the propagation of said beam outside said enclosure.

For a better understanding of the invention and to show how the same may be carried into effect, reference will be made to the drawings given solely by way of example, which accompany the following description.

In the drawings FIGS. 1 and 2 respectively illustrate, in longitudinal and transversal sectional views, a vacuum tube in accordancewith the invention FIG. 3 illustrates in detail the attachment system of thewire cathode FIG. 4 illustrates an elongated cathode provided with electrodes for focussing and controlling the uniformity of the beam, in accordance with the invention FIG. 5 illustrates in detail the control electrode and FIG. 6 illustrates, in section, the window of the evacuated enclosure, in accordance with the invention.

In FIG. I, an evacuated enclosure 1 of cylindrical form has been illustrated, containing a cathode made up of two tungsten filaments 2 and 3 (only the filament 2 being visible in FIG. 1).

These two filaments are arranged one beside the other, parallel to the axis of the tube and are attached on the one hand, to the base 4 of the enclosure 1, through which they pass, an insulating material 5 being provided to electrically insulate each filament from the metal base 4 of the enclosure 1, and, on the other hand, to a bracket 7 of insulating material, movable about a shaft 8. A tension spring 9 attached to an end 10 of the bracket, on the one hand,and to a bar I] fixed to an electrode 12, on the other, suitably tensions the wire cathodes 2 and 3.(as shown in FIG. 3).

Along this double wire cathode, there are arranged electrodes 12, 13, 14 which are concentric and insulated from one another. The electrode 12 called control electrode is shown in FIGS. 4 and 5. This electrode 12 controls the form and the homogeneity of the electron beam. It is constituted withseveral electrode elements which are substantially cylindrical similar and coaxial metal sleeves 15, 16, 17 and 18 arranged in succession and electrically insulated from one another by means insulating rings r. These sleeves and rings are provided with openings m,n, running along the cathode. A voltage is respectively applied to. each of these sleeves 15, 16, 17 and 18 by means of connections 19, 20, 21 and 22 these voltages are separately adjustable, enabling accurate control of the focussing and uniformity of the beam to be achieved.

These sleeves 15, 16, 17-, 18 are generally negatively biased in relation to the cathode and therefore carry only a very weak current. Two accelerator anodes l3 and 14 of cylindrical form, concentric with the control electrode 12 present a longitudinal opening, opposite the wire cathodes 2 and 3, for the passage of the beam. These anodes are respectively connected to two separately controllable voltage sources by means of conductors 23 and 24, as shown in FIG. 1.

In one possible embodiment, this kind of generator comprises a cathode made up of two parallel tungsten wires 50 cm long, this cathode operating in the saturation condition (due to the space charge), so that the disuniformities in the cathode emission are smoothed out the control electrode 12, made up, for example, of. four sleeves each 8 cm wide separated from one an other by 5 mm wide rings r of insulating material, enable fine correction requiring a voltage of between one and two percent of the total voltage, namely 4 to 8 kV, to be carried out. This kind of system, requiring four control voltages which are isolate-d from one another, can utilise a high-voltage source of the type employing cascades of transformers with a single primary.

In order to produce a vacuum tube in accordance with the invention, whose structure is both simple and robust and enabling this tube to operate continuously for several hundreds of hours, it may be desirable to use the technique of sealing the tubes to a titanium pump, argon-arc welds making easy the opening of the vacuum tube, if necessary. The window requires particular care in this kind of design.

The window can be made of a very thin rectangular strip, of thickness substantially equal to 0.1 mm if made of aluminum 0.05 inches if made of stainless steel 0.05 inches if made of titanium 0.1 inches if made of oxyde of beryllium (or glucine).

Its length can range between 0.5 and 1 metre and its width is substantially 3 cm (the beam in this example) having a width of about 2 cm). This window must be able to dissipate about 20 to 40 watts per cm But it should be pointed out that these wide windows are more brittle than the usual windows of linear accelerators where the power density, which moreover can be higher at the window is furnished by a smalldiameter cylindrical beam, allowing the heat to be transmitted to the periphery of the plate, whilst, in the generator in accordance with the invention, the window, which is wide, only effectively transmits the heat to two sides.

This kind of window, if cooled solely by conduction, does not enable a current density in excess of 50 [.LA/Cl'fl to be used.

if air circulation is used for cooling, then there is some risk of chemical attack of the material of the window. This is why, in the tube in accordance with the invention, an inert cooling gas under forced circulation is used, the base being purified nitrogen or argon.

FIG. 6 illustrates, in transverse section, a window of the kind used in the tube. This window is constituted by a first thin strip 25, of 0.03 mm thick, for example, which is electron-transparent and is 50 cm long and 3 cm wide. This first strip 25 is welded by electron bombardment to a copper base 26 which is brazed to a frame 27 of stainless steel. This frame 27 is is turn attached to a metal block 29 of rectangular form by argon arc welding 28, the dimensions of said block 29 being larger than those of the window. The metal block 29 is fixed to the evacuated enclosure 1 by means of a brazing 30. A cut-out 31 machined in the block 29 enables the window assembly to withstand the distortions due to the difference of the pressure on both sides of said window.

In front of the first strip 25, a second strip 32 which is electron-transparent, is arranged. This second strip 32 is an aluminum strip between and a thick. It is arranged between two metal components 33 and 34 (made of stainless steel for example) and held in a vacuumtight fashion between thesetwo components by means of a set of screws 35. These strips and 32 are separated from each other by a space s. The metal component 34 is itself attached to the block 29 by means of screws 36. A cooling system, not shown in the figure, enables an inert purified cooling gas (nitrogen or argon for example) to be circulated through the space s defined between the first strip 25 and the second strip 32.

The vacuum-tight closure providedwith this window is robustand enables the enclosure to be opened repeatedly.

The titanium first strip can be replaced, of course, by a beryllium oxide strip, a material well known for its excellent thermal conductivity however, a large-sized window is difficult to produce in this material.

What I claim is l. A vacuum tube for generating a wide beam for electrons, comprising a vacuumtight enclosure in said enclosure at least one elongated cathode for emitting an electron beam throughout its length, a control electrode for controlling the form and the homogeneity of said beam, said electrode comprising an elongated gap along which said cathode extends and through which said beam propagates said control electrode comprising a plurality of electrode elements electrically insulated from one another means for respectively connecting said elements to a voltage supply means for accelerating, said beam said vacuum-tight enclosure comprising an elongated electron-transparent window for the propagation of said beam outside said enclosure.

2. A vacuum tube as claimed in claim 1, wherein said electron-transparent window comprises thin first and second strips, supporting block for supporting said strips, said block being integrally connected with said vacuumtight enclosure said strips being separated from each other thus defining a space said cooling system comprising means for circulating an inert purified gas within said space.

3. A vacuum tube as claimed in claim 2, wherein said first strip is of titanium.

4. A vacuum tube as claimed in claim 2, wherein said first strip is of oxyde of beryllium.

5. A vacuum tube as claimed in claim 2, wherein said second strip is of aluminum.

6. A vacuum tube as claimed in claim 5, wherein said connecting means comprise means for applying respectively different voltages to said elements.

7. A vacuum tube for generating a wide beam for electrons, comprising a vacuumtight enclosure in said enclosure at least one elongated cathode for emitting an electron beam throught its length, a control electrode for controlling the form and the homogeneity of said beam, said electrode comprising an elongated gap along which said cathode extends and through which said beam propagates said control electrode comprising a plurality of electrode elements electrically insulated from one another means for respectively connecting said elements to a voltage supply means for accelerating said beam said vacuum-tight enclosure comprising an elongated electron transparent window for the propagation of said beam outside said enclosure said control electrode elements being substantially cylindrical similar and coaxial metal sleeves provided with a longitudinal opening, said openings forming said gap said sleeves being insulated from each other by means of insulating rings, coaxial with and having an opening substantially coextensive with said sleeve openings. 

1. A vacuum tube for generating a wide beam for electrons, comprising a vacuumtight enclosure ; in said enclosure at least one elongated cathode for emitting an electron beam throughout its length, a control electrode for controlling the form and the homogeneity of said beam, said electrode comprising an elongated gap along which said cathode extends and through which said beam propagates ; said control electrode comprising a plurality of electrode elements electrically insulated from one another ; means for respectively connecting said elements to a voltage supply ; means for accelerating, said beam ; said vacuum-tight enclosure comprising an elongated electron-transparent window for the propagation of said beam outside said enclosure.
 2. A vacuum tube as claimed in claim 1, wherein said electron-transparent window comprises thin first and second strips, supporting block for supporting said strips, said block being integrally connected with said vacuumtight enclosure ; said strips being separated from each other thus defining a space ; said cooling system comprising means for circulating an inert purified gas within said space.
 3. A vacuum tube as claimed in claim 2, wherein said first strip is of titanium.
 4. A vacuum tube as claimed in claim 2, wherein said first strip is of oxyde of beryllium.
 5. A vacuum tube as claimed in claim 2, wherein said second strip is of aluminum.
 6. A vacuum tube as claimed in claim 5, wherein said connecting means comprise means for applying respectively different voltages to said elements.
 7. A vacuum tube for generating a wide beam for electrons, comprising a vacuumtight enclosure ; in said enclosure at least one elongated cathode for emitting an electron beam throught its length, a control electrode for controlling the form and the homogeneity of said beam, said electrode comprising an elongated gap along which said cathode extends and through which said beam propagates ; said control electrode comprising a plurality of electrode elements electrically insulated from one another ; means for respectively connecting said elements to a voltage supply ; means for accelerating said beam ; said vacuum-tight enclosure comprising an elongated electron transparent window for the propagation of said beam outside said enclosure ; said control electrode elements being substantially cylindrical similar and coaxial metal sleeves provided with a longitudinal opening, said openings forming said gap ; said sleeves being insulated from each other by means of insulating rings, coaxial with and having an opening substantially coextensive with said sleeve openings. 